Atomic Weight

weight of one atom of a particular element (one unit of weight is equal to 1/12 of the weight of a carbon-12 atom)

The atomic mass (ma) is the mass of an atom at rest, most often expressed in unified atomic mass units. The atomic mass may be considered to be the total mass of protons, neutrons and electrons in a single atom (when the atom is motionless). The atomic mass is sometimes incorrectly used as a synonym of relative atomic mass, average atomic mass and atomic weight; however, these differ subtly from the atomic mass. The atomic mass is defined as the mass of an atom, which can only be one isotope at a time and is not an abundance-weighted average. The actual numerical difference is usually very small such that it does not affect most bulk calculations but such an error can be critical when considering individual atoms.
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Noun
1. (chemistry) the ratio of the atomic mass of an element to half the atomic mass of carbon-12
(synonym) relative atomic mass
(hypernym) mass
(hyponym) equivalent, equivalent weight, combining weight, eq
(classification) chemistry, chemical science

the weight of an atom of a specific element where one atomic weight unit is defined as the weight of 1/12 of a carbon 12 Atom.

The average weight (or mass) of all the isotopes of an element, as determined from the proportions in which they are present in a given element, compared with the mass of the 12 isotope of carbon (taken as precisely 12.000), that is the official international standard; measured in daltons.